Copyleft licensing was invented in an era when most things were written in C, and software fell into roughly four categories:

• Unix-ish black-box "primitive" tools, that were so focused on accomplishing one fundamental job that they were essentially "final" in their interfaces, with there being no point to extending them any further; where you reused them by executing them, not by integrating with them.

• A library for such Unix-ish black-box tools to use. Most tools that used any libraries at all, would use one main library to accomplish their one primary purpose, effectively making the tool a "driver program" for the library.

• Academic data-science/statistics code.

• Cathedral-style highly-integrated software, e.g. Windows.

Copyleft was mostly devised for the purpose of licensing the codebases of the first two types.

As Unix tools are self-contained, they only "infect" direct forks. The GPL originally intentionally avoided infecting the things that called (i.e. interacted with) those tools — because, back then, a downstream project that "uses" a tool wasn't vendoring in its own version, but rather relying on the system installation of that tool, through that tool's known API; and it wouldn't make sense for a license to be infectious through a standardized API.

It was intentional that libraries would infect their downstream clients with copyleft; but downstream clients, back then, were mostly just those single-purpose tools. It wouldn't make sense for e.g. libgit to be GPL-licensed, but for git(1) to be proprietary.

Of course, there was also an awareness that the Cathedral-style codebases would have their whole monolith infected if they used the GPLed library. The idea there, though, wasn't to actually cause that infection — it was to inhibit Cathedral-style codebases from using GPLed software at all.

(With the parallel awareness that such entities could always reach out to the project maintainers, and buy a separate license, just like you can purchase a license from any IP-holder. There were few-enough contributors per project, back then, that "copyright assignment" and such wasn't a concern; you could just get a proprietary license from the one dude who built the whole thing.)

And that same consideration was implicit with academic use of GPLed software. FOSS programmers, back then, considered academic (or non-profit) integration/derivation of their software to be something they'd grant a free license for if asked; and academics knew this, and so didn't bother to ask for such licenses, because they knew they'd almost assuredly get one, for free, if-and-when it ever became important to do so.

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The GPL was well-suited to this early-90s software IP ecosystem. It doesn't fit nearly as well in the modern software IP ecosystem.

There's a whole fifth category of software — semi-Cathedral, semi-Bazaar mega-tools, like youtube-dl or Calibre; or mega-libraries, like Qt, or LLVM, or WebKit; which both are components while also consuming many components themselves. The GPL never "expected" this type of software. This kind of software just didn't exist back then; only its entirely-Cathedal final-product equivalent did.

Which is a problem, because it's impossible to build something like WebKit or LLVM in a self-contained, "you call it over a standard interface" sort of way, where it's non-infectious. These days, lot more projects are infectious, even when "integrated" at a much higher level of abstraction, than the GPL was ever intended to require.